1. Technical Field
The present invention relates to the supply of control voltages for an inverter. For example, the inverter is designed to feed an induction motor at adjustable speed and torque or to generate output sine wave voltages of an uninterruptible power supply. More generally, the invention supplies parallel sequences of control signals from a digital microcontroller.
2. Background Art
FIG. 1 shows the conventional diagram of a three-leg inverter controlled by pulse width modulation (PWM) to supply an asynchronous three-phase motor M. The motor M comprises three inductors L1, L2, L3. In order to run the motor, it is necessary to apply to the inductors sine wave voltages U, V and W, 120.degree. shifted with respect to the other, as shown in FIG. 2. Voltages U, V, W are provided by an inverter comprising six switches T1-T6, for example transistors. Transistors T1-T2, T3-T4, T5-T6 are connected in series between the positive and negative (for example the ground) supply terminals. In FIG. 1, each switch T1-T6 comprises a power switch associated to an anti-parallel diode D1-D6. The control electrodes of two switches in series are oppositely controlled. Thus, it is possible, by properly controlling the control terminals G1-G6 of switches T1-T6 to provide at the junctions U, V and W of switches T1-T2, T3-T4, T5-T6 stage voltages approximating sine waves. More particularly, by varying the duty cycles of switches T1 and T2, (for example by successive steps ranging from 0 to 100% or from 40 to 60%), the average voltage at junction U is made to vary by successive steps to produce sine waves, the amplitude of which is controlled. The period, NT, of the sine waves will be determined by the time period T of each step and the number of steps N. For example, the number N will be chosen equal to 24.
FIG. 3 shows exemplary signals applied to the control terminals G1-G6 of transistors T1-T6 during the time period T of a step. In this example, the duty cycle of signal G1 is 90%, that of signal G3 is 50% and that of signal G5 is 10%. The duty cycles of signals G2, G4, G6 are complementary. During the next step, the time period of these duty cycles is changed, and so forth until the complete period (NT) has elapsed.
The aim of the invention is to provide a method for supplying these sequences of control signals. This problem is for example dealt with in the article by Bellini et al. published in "International Conference on Evolution and Modern Aspects of Induction Machines--Proceedings", Jul. 8-11, 1986, pages 194-202. As indicated in this article, analog and digital techniques have been devised to supply these signals.
A conventional digital system for supplying these signals is shown in FIG. 4. The system mainly uses a microcontroller (MCU) 10 which, for each step, provides the values of the duty cycles of the various bridge legs of FIG. 1. For each step, the MCU provides three digital values and eventually their complements. The microcontroller is followed by a specific interface circuit (ASIC) 11 especially designed to rapidly calculate and supply the control signals shown in FIG. 3 derived from the digital duty cycle values. The six outputs of ASIC 11 are provided to amplification and isolating interface circuits 12 which access the control terminals of the six transistors T1-T6.
This type of arrangement has various drawbacks. First, two integrated circuits are required for providing the control signals, namely MCU 10 and ASIC 11. ASIC 11 is a circuit especially designed for this application and is therefore not manufactured in mass production, which unavoidably entails a relatively high cost. Furthermore, the ASIC 11 is adapted to a specific application; and it is not programmable. The ASIC 11 therefore must be redesigned to change its functions, for example, as will be explained later on, to modify the distribution of the control signals.